Certain vehicle seat modules control the air pressure inside multiple bladders. One or more of the bladders is located under the surface of a vehicle seat. The contour of the seat is modified with changing pressure. Certain modules may employ air valve technology which internally uses a shape memory alloy (SMA) wire to actuate the valve. When heated, the SMA property causes the wire to shrink and thereby open the valve.
The valve technology includes a limit switch. The limit switch is used to sense when the valve is in its full opened position. The valve is in its full opened position while the wire is adequately heated such that the wire is shrunk enough to thereby open the valve to its full opened position. The limit switch is closed when the valve is in its full opened position. As such, by detecting that the limit switch is closed it can be detected that the valve is in its full opened position. The valve is not in its full opened position while the wire is not adequately heated. The limit switch is not closed (e.g., the limit switch is opened) when the valve is not in its full opened position. As such, by detecting that the limit switch is opened it can be detected that the valve is not in its full opened position.
Using the limit switch to sense the position of the valve permits control circuits to maintain the valve in its full opened position while applying further minimal heat to the wire. This level of control provides predictable behavior of the valve over varying temperature and air flow conditions and also minimizes the heat and mechanical stress on the wire.
Pulse Width Modulation (PWM) may be used for heating the wire. Current pulses are passed through the wire with resistivity losses of the wire causing self-heating of the wire. The PWM current pulses can be generated by a microcontroller for application to the wire.